A plastic lens is lighter than an inorganic lens, is not easily broken, and is dyeable. Therefore, the plastic lens has become rapidly widespread as an optical element such as a spectacle lens or a camera lens. Up to now, various resins for spectacle lenses have been developed and used. Typical examples of the resins include an allyl resin obtained from diethylene glycol bisallyl carbonate or diallyl isophthalate, a (meth)acryl resin obtained from (meth)acrylate, and a thiopolyurethane resin obtained from isocyanate and thiol.
Recently, a polyurethane resin comprised of isocyanate and an alcohol has been developed (Patent Documents 1 to 3).
Generally, the compatibility between isocyanate and an alcohol is poor. Accordingly, the process for producing a polyurethane resin described in Patent Documents 1 to 3 employs a method of reacting isocyanate with a portion of the alcohol first (hereinafter, this step will be referred to as “prepolymerization”) and then reacting the prepolymer with remaining alcohol. Generally, the prepolymerization is performed by heating. Moreover, because the prepolymer as a reactant has high viscosity, the reaction between the prepolymer and remaining alcohol is also performed by heating so as to improve workability and compatibility.
For example, in paragraph [0233] of Examples of Patent Document 1 (PCT Japanese Translation Patent Publication No. 2009-520057), the conditions of the prepolymerization are described as below.
“4,4′-Methylene bis(cyclohexylisocyanate) (1.0 molar equivalent) is heated to 70° C., 1,1,1-tris(hydroxymethyl) propane (0.2 molar equivalents) is then added thereto, and the reaction temperature is maintained at a temperature of less than 120° C.”
Furthermore, viscosity at 73° C. of prepolymers 1, 2, and 3 obtained in Example is high, such as 90 mPa·s, 2,103 mPa·s, and 7,000 mPa·s respectively (Table 1). Therefore, according to the documents, these prepolymers are heated to 70° C., 70° C., and 80° C. respectively so as to improve workability at the time of handling the prepolymers.
Paragraph [0054] of Detailed Description of Patent Document 2 (Japanese Unexamined Patent Publication No. 2008-144154) describes that the prepolymerization is performed under the condition in which “it is heated in a reaction container at a temperature of, for example, room temperature to 150° C. and preferably at a temperature of 50° C. to 120° C.” Moreover, paragraph [0078] of Examples of the document describes that the prepolymerization is performed under the condition in which “reactants are reacted at 90° C. for 4 hours.”
In paragraph [1086] of Examples of Patent Document 3 (US 2009/0281268 A1), the condition of the prepolymerization is described as below.
“An isocyanate functional urethane prepolymer was prepared by reacting 0.3 equivalents of 1,5-pentanediol, 1.0 equivalent of DESMODUR W 4,4′-methylene-bis-(cyclohexyl isocyanate) and 5 ppm dibutyltin diacetate as reactants. The DESMODUR W and 1,5-pentanediol were each preheated to a temperature of about 80° C. under a nitrogen atmosphere for at least about four hours prior to mixing.”
In addition, Table 27 of the document describes that a tank temperature of the prepolymer is maintained at a high temperature of 110° C. to 127° C.
In Patent Document 4 (Japanese Unexamined Patent Publication No. 2002-187931), the viscosity of a urethane prepolymer having various isocyanato groups is described. According to the document, the viscosity at 30° C. of the prepolymer is high such as 2,200 mPa·s to 8,600 mPa·s.